Tracking the position of probes within the body is required for many medical procedures. For example, various systems have been developed for determining the position coordinates (location and/or orientation) of an object in the body based on magnetic field sensing. These systems use sensors affixed to the object to measure the relative strengths of externally-generated magnetic fields and to derive from these measurements the position of the object. (The term “position” as used in the present patent application and in the claims refers to any set of spatial coordinates, including either location coordinates, angular orientation coordinates, or both.) Methods for magnetic-based position sensing are disclosed, for example, in U.S. Pat. Nos. 5,391,199, 5,443,489, and 6,788,967 to Ben-Haim, in U.S. Pat. No. 6,690,963 to Ben-Haim, et al., in U.S. Pat. No. 5,558,091 to Acker et al., in U.S. Pat. No. 6,172,499 to Ashe, and in U.S. Pat. No. 6,177,792 to Govari, all of whose disclosures are incorporated herein by reference.
When accurate position measurements are required, the probe may be calibrated in advance. An exemplary calibration process is described in U.S. Pat. No. 6,266,551 to Osadchy et al., whose disclosure is incorporated herein by reference. In the embodiments described in this patent, a device used to determine the location and orientation of a catheter inside the body comprises a plurality of coils adjacent to the distal end of the catheter. The catheter further comprises an electronic microcircuit adjacent to the proximal end of the catheter, which stores information relating to the calibration of the device. The microcircuit comprises a read/write memory component, such as an EEPROM, EPROM, PROM, Flash ROM or non-volatile RAM, and the information is stored in digital form. The calibration information includes data relating to the relative displacement of the distal tip of the catheter from the coils. The calibration information may also include data relating to deviation of the coils from orthogonality, or data relating to the respective gains of the coils, or a combination of these data.
U.S. Pat. No. 6,370,411 to Osadchy et al., whose disclosure is incorporated herein by reference, describes a catheter assembly for connection to a control console. The catheter assembly comprises two parts: a catheter of minimal complexity which is inserted into a patient's body, and a connection cable which connects between the proximal end of the catheter and the console. The catheter comprises a microcircuit which may carry calibration data that is specific to the catheter. The cable comprises an access circuit, which receives the information from the catheter and passes it in a suitable form to the console. Preferably, the cable operates with all catheters of a specific model or type, and therefore when a catheter is replaced, there is no need to replace the cable. The cable comprises an additional microcircuit in which information characteristic of one or more models of catheters associated with the cable is stored. The additional microcircuit may also include calibration information for the access circuit and amplifiers within the cable. The calibration information of the amplifiers may include, for example, their zero-gain, DC offset and linearity.